An internal investigation at the interior department has concluded that secretary Ryan Zinke abused his office by allowing family members to ride with him in government vehicles, in violation of department regulations prohibiting that practice.

An inspector general report said that Zinke had invited his wife Lolita Zinke on certain boat travel with a green light from his department’s solicitor general’s office. But in other cases, the Zinke family had reimbursed the department for family travel, investigators found.

The report also found that the department had paid $25,000 for a security detail to accompany the Zinke family on a summer vacation in Turkey and Greece.

The report found that Zinke’s staff had researched whether Lolita Zinke could be classified as a department volunteer, allegedly to circumvent a requirement for the family to reimburse the department for her travel. Zinke denied that the volunteer idea was related to Lolita Zinke’s travel expenses. She never was classified as a volunteer, the report found.

An interior department spokeswoman, Heather Swift, dismissed the report in a statement.

“The Inspector General report proves what we have known all along: the secretary follows all relevant laws and regulations and that all of his travel was reviewed and approved by career ethics officials and solicitors prior to travel,” she said in an email to the Washington Post, which first reported on the investigation.

The report landed in a turbulent period for the department, which has struggled this week to explain the installation of a new official in the department’s inspector general’s office. In addition to the investigation of Zinke’s travel, the department has three other active investigations of Zinke’s conduct in his post.

The Trump administration had reportedly moved to rotate Suzanne Israel Tufts, a political appointee previously with the Department of Housing and Urban Development (Hud), into an acting role as interior department inspector general earlier this week.

But Swift, the interior spokesman, denied that Tufts was moving to the department. “Ms Tufts is not employed by the Department and no decision was ever made to move her to Interior,” Swift said in a statement, adding that a Hud announcement to the contrary was “100 percent false information”.

The report on the Zinkes’ travel found that the family had reimbursed costs of travel “where appropriate”.

Certain allegations investigated by the inspector general were determined to be unfounded. For example, investigators looked into an allegation “that a DOI employee resigned because Secretary Zinke made her walk his dog while at work”.

“According to the former employee, however,” the report found, “Secretary Zinke never told her to walk his dog at work. She said she often volunteered for the task and was happy to do it because she liked the dog.

“Secretary Zinke said that he occasionally brought his dog to the office and that employees volunteered to walk the dog, although he typically walked the dog himself.”